As is widely known in the imaging arts, a thermographic imaging process relies on the use of heat to help produce an image. Typically, a thermally sensitive image forming layer is coated on top of a suitable base or substrate material such as paper, plastics, metals, glass, and the like. The resulting thermographic construction is then heated at an elevated temperature, typically in the range of about 60.degree.-225.degree. C., resulting in the formation of an image. Many times, the thermographic construction is brought into contact with the thermal head of a thermographic recording apparatus, such as a thermal printer, thermal facsimile, and the like. In such instances, an anti-stick layer is coated on top of the imaging layer in order to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized.
Thermographic materials whose image forming layers are based on silver salts of long chain fatty acids, such as silver behenate, are known. At elevated temperatures, silver behenate is reduced by a reducing agent for silver ion such as hydroquinone, substituted hydroquinones, hindered phenols, catechol, pyrogallol, methyl gallate, leuco dyes, and the like, whereby an image is formed.
It is also known that other additives can be added to imaging layers of thermographic constructions to enhance their effectiveness. For example, U.S. Pat. No. 2,910,377 discloses that the silver image for such materials can be improved in color and density by the addition of toners to the imaging layer. Toners which give primarily image density enhancement are also referred to as development accelerators.
U.S. Pat. No. 3,080,254 discloses the use of phthalazinone as a toner in heat-sensitive copying paper. U.S. Pat. No. 3,847,612 discloses an improved imaging system containing an imidazole in combination with phthalic acid and the like. Phthalazine in combination with phthalic acid and other organic acids also provide an improvement in image formation. Such disclosed combinations are particularly valuable when relatively weak reducing agents, such as hindered phenols, are used as the developer for silver soaps.
U.S. Pat. No. 4,585,734 discloses the achievement of good toning when a combination of phthalazine and an active hydrogen-containing heterocyclic compound such as phthalimide, naphthalimide, pyrazole, and succinimide are employed in dry silver imaging systems.
Imaging systems which contain active ingredients that increase the thermal sensitivity and image forming capabilities of thermographic constructions are continuously needed in the imaging arts.